1. Field of the Invention
This invention relates to a trench power metal oxide semiconductor field effect transistor (MOSFET) structure and a method to fabricate the same, and more particularly relates to a closed cell trench power MOSFET structure and a method to fabricate the same.
2. Description of the Prior Art
Attending with the developing of high frequency circuit applications, the demand of high switching speed transistors is increasing. The transistor structure with higher switching speed is capable to reduce switching loss so as to enhance power efficiency.
FIG. 1A is a schematic view of a typical striped cell trench power MOSFET structure. As shown, the trench power MOSFET structure has an N-type heavily doped substrate 10, a N-type epitaxial layer 12, a P-type body 13, a gate oxide layer 15, a trenched gate 16, and a plurality of source regions 17. The epitaxial layer 12 is formed on the substrate 10. The body 13 is formed on the epitaxial layer 12. The gate oxide layer 15 encircles the trenched gate 16 to separate the trenched gate 16 and body 13. The trenched gate 16 is composed of a plurality of stripe-shaped polysilicon structures, which are located in the body 13 with a predetermined interval. A bottom of the stripe-shaped polysilicon structure is located in the epitaxial layer 12 below the body 13. The source regions 17 located by the both sides of the stripe-shaped polysilicon structures.
FIG. 1B is a schematic view of a typical closed cell trench power MOSFET structure. As shown, the trench power MOSFET structure has an N-type heavily doped substrate 20, a N-type epitaxial layer 22, a P-type body 23, a gate oxide layer 25, a trenched gate 26, and a plurality of source regions 27. The epitaxial layer 22 is located on the substrate 20. The body 23 is located on the epitaxial layer 22. The gate oxide layer 25 encircles the trenched gate 26 to separate the trenched gate 26 and the P body 23. The trenched gate 26 shows a network structure in the body 23 to define a plurality of square areas. The bottom of the trenched gate 26 is located in the epitaxial layer 22 below the body 23. The source regions 27 are located in the square areas defined by the trenched gate 26.
The channel of the closed cell trench power MOSFET structure has a width proportional to the boundary length of the square source regions 27. The channel width of the striped cell trench power MOSFET structure is proportional to a side length of the stripe-shaped source regions 17. In contrast with the striped cell one, the closed cell trench power MOSFET structure featuring a greater channel width per unit surface area has a lower on-resistance (Ron).
However, as gate-to-drain capacitance (Cgd) is concerned, because gate-to-drain capacitance is proportional to the bottom area of the trenched gate 16,26, the trenched gate 26 of the closed cell trench power MOSFET structure occupies a greater surface area than the striped cell one may result in a higher gate-to-drain capacitance.
In conclusion, the closed cell trench power MOSFET structure features lower on-resistance but higher gate-to-drain capacitance in contrast with the striped cell one. The feature of high gate-to-drain capacitance may restrict the switching speed of the transistor structure and hinder the development of high frequency electronic circuit applications. Accordingly, it has become an important issue in the field of the present invention to improve gate-to-drain capacitance of the closed cell trench power MOSFET structure.